Many digital communication systems like wireless systems and cable modem systems use hybrid automatic gain control circuits with an analog portion and a digital portion. These systems do calibration and error control in the digital circuitry, and apply the gain correction derived by the digital circuitry to the analog gain control circuitry.
The headend transceiver in DOCSIS cable modem systems, must be able to receive transmissions from many different cable modems at different distances. Each cable modem sends bursts of data that are quadrature amplitude modulated at a power level that is specified by the headend transceiver. It is important to control the gain of the received signal so that it does not saturate the front end analog circuitry and so that the received signal from each cable modem falls within the dynamic range of analog-to-digital converters which digitize the received signals. The amplitude of each received constellation point is an important piece of information since the position of each constellation point in the constellation of possible points that can be transmitted is controlled by both the point's amplitude and phase. These two coordinates control the position, and the position represents the digital bits that were sent when that constellation point is received.
However, during normal DOCSIS operations, the headend transceiver controls the transmit power of the cable modems so that their bursts arrive at a nominal power level within the dynamic range of the headend receiver's A/D converter. During normal operation, there is no need for an automatic gain control of the type disclosed herein.
Headend receivers in cable modem systems typically have a front end comprised of an analog attenuator that receives signals from the hybrid fiber coax and outputs a signal which has been attenuated by a specific amount to an A/D converter. The amount of attenuation by the analog attenuator is controlled by an analog control voltage input. Each analog attenuator applies a different amount of attenuation based upon the same control word because of variations in the manufacturing process from one lot to the next. How much attenuation the analog attenuator imposes for each different level of control word voltage must be known before the headend transceiver is placed into service. This is important so that the cable system operator will be able to precisely control the attenuation by applying the appropriate control voltage. A system for generating an attenuation table that can be used to generate the proper control voltages to cause a specified amount attenuation to be imposed during actual operation is therefore needed. This creates a need for a control word generator that can be used during manufacture to generate a control word table that contains the proper control word for each desired level of attenuation.